PHP mutliple .scss Files to one .css File after change

Question

i would like to compile multiple .scss files to one .css file.

my code:

<?php
class SassCompile {

private $scssFileType = ".scss";
private $cssFileType = ".css";
private $cssFileName = "style";

public $scssFolder = "scss/";
public $cssFolder = "css/";

public $scssPhp = "scssphp/scss.inc.php";
public $scssPhpCompression = "scss_formatter_compressed";

public function compile($scss_folder, $css_folder, $scssphp, $scss_format) {
    require $scssphp;

    $scss_compiler = new scssc();
    $scss_compiler->setImportPaths($scss_folder);
    $scss_compiler->setFormatter($scss_format);


    $filelist = glob($scss_folder . "*" . $this->scssFileType);
    foreach ($filelist as $file_path) {
        $file_path_elements = pathinfo($file_path);
        $file_name = $file_path_elements['filename'];
        $file_content = file_get_contents($scss_folder . $file_name . $this->scssFileType);
        $string_css = $scss_compiler->compile($file_content);
        file_put_contents($css_folder . $this->cssFileName . $this->cssFileType, $string_css);
    }
}

}

$sassCompile = new SassCompile();
$sassCompile->compile($sassCompile->scssFolder, $sassCompile->cssFolder, $sassCompile->scssPhp, $sassCompile->scssPhpCompression);

my import.scss looks like

@import "style";
@import "test";

i have two files in my scss folder: "_style.scss" and "_test.scss". these files will be compilet to my stle.css in the css folder.

Everytime the page is reloaded, the css will be compiled. When i have more users, it will kill my webserver.

How can i add a function to only compile if the file has changes since the latest compile was done?

thank you